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Most likely down to your routing. The UDAC8 is a simple 8ch DAC. if your player isn't configure to route audio to these extra channels, maybe an issue. Windows driver aren't the one used on the system, we use ASIO as I'm sure you already know?

Fixed: I configured Asio UDAC8 in foobar correctly (I had not clicked on "Apply" ..... sigh).
Now I can drive 4 different speakers (subwoofer, woofer, midrange, tweeter).
The yield is now really excellent